(601c) Computational Studies of the Dry Reforming Reaction on Al2O3-Coated Transition Metal Catalysts

More than 50 percent of man-made greenhouse gases are composed by carbon dioxide (CO₂), which makes urgent the development of effective strategies for remediation of CO₂ emissions. A promising route for chemical conversion of CO₂ is the dry reforming reaction (DRR) between CO₂ and methane (CH₄) to produce Syngas. Syngas is a mixture of H₂ and CO used as intermediate production of synthetic natural gas and ultra-clean fuels, or as precursor for ammonia, methanol, and other valued-added chemicals. Thus, the DRR reaction opens a window for remediation processes of CO₂ with high promise of producing value-added chemicals.

So far the screening of catalysts has shown that metallic catalysts from Group VIII are active for the DRR reaction. Among these catalysts the Rh-based seem to be the most active but the Ni-based catalysts have the best compromise between performance and cost. However, all of these catalysts suffer of sintering and coke formation due to the high temperature required for the DRR reaction (~800 °C), which still hinders the implementation at commercial scale of this route for chemical conversion of CO₂. In case of Ni-based catalysts two main causes are associated to sintering and coke formation. The first cause is the inability to precisely control the synthesis parameters and the interactions between the active site and the surface acting as support. The second cause is the exposure of the active catalytic sites to the extreme temperature at which the DRR reaction takes place. While the exposure of the catalysts to the high temperatures at which the DRR is performed is necessary, the minimization of sintering and coke formation can be performed through development of structures able to control the continuous structural reconstruction of the active phase.

Sintering and coke formation can be minimized by synthesis of Ni nanoparticles (NPs) surrounded by atomically precise assemblies protecting the active site. Recent experiments have reported increased resistance to sintering and coke formation for Pd-based catalysts overcoated with an Alumina layer using atomic layer deposition (ALD) methods. It was suggested that this Alumina layer may offer better protection to the active phase thanks to stabilization of the Ni nanoparticles. Those Pd catalysts covered by the Alumina layer resulted in only 5% of carbon accumulation in comparison with bared Pd catalysts, indicating increased resistance to catalyst coking as the ALD overcoating is applied. However, there is still debate regarding the effect of overcoating thickness on catalysts activity, as thinner coatings have been also associated with better overall catalytic performance.

So far the above mentioned experiments have been performed using structures composed of monometallic Pd NPs deposited on non-modified Al₂O₃ substrates without additives to stabilize the overcoating ALD AL₂O₃ structure. However, additional improvements on these structures can potentially enhance the catalyst stability and selectivity. In this work we examined a new class of Ni catalysts coated with Alumina using ALD methods. The reasons for the enhanced catalytic efficiency were investigated through state-of-the-art DFT/AIMD analysis. The active phases of Ni nanoparticles were represented by terrace and stepped slabs in order to evaluate the earlier steps of the Alumina layer formation. It is shown that the Alumina overcoating of Ni nanoparticles is viable but also Alumina deposition is preferred on stepped regions, leaving available the catalytically active terrace regions. The effects of Alumina overcoating on the mechanistic pathways for CO₂ activation were also studied, unveiling the effect of shape changes and nanoparticles reconstruction under support-nanoparticle and nanoparticle-overcoat interactions, and the possible appearance of side reactions and coke formation.